Sieving apparatus

ABSTRACT

An industrial sieving or screening machine has a resonator rod on the separator screen. The rod extends between two spaced ends and has a transducer located at one end to excite the rod over its length to assist with deblinding the screen. The rod may be spiral in shape or shaped with other smoothly blended complex curves such as an S-shape. A spiral rod resonator fixed to the top of the sieve screen may be used as a guide for material to be screened.

FIELD OF THE INVENTION

The present invention relates to sieves both for dry particulate solidsand for liquids and particularly sieves in which an excitation sourceprovides deblinding excitation of the sieve screen.

BACKGROUND OF THE INVENTION

Most industrial sieving machines include some means of applying aprimary vibratory movement to the sieving screen in order to facilitateproduct movement through the screen and also to create a flow ofmaterial over the screen surface. This ensures maximum utilisation ofthe active screening area and that oversized product can be transportedto an outlet to be removed. The primary vibratory movement is often acombination of horizontal and vertical reciprocating motion which maytypically be applied to the frame carrying the sieve mesh or screen in avariety of ways, such as by rotating out-of-balance weights, or a directdrive by a rigid crank or cam system.

A problem with sieving machines is blinding of the screen, particularlywhen sieving damp or sticky materials. Blinding is a significant problemin the industrial sieving of certain powders and also in the strainingof liquids. To overcome the blinding problem secondary vibrations,preferably flexural, have been applied to the screen, for example byimpacts from deblinding discs or the application of high and ultrasonicfrequencies (see for example EP-A-0369572).

Typical ultrasonic frequencies are above 20 kHz, and typical amplitudesof the ultrasonic vibration supplied to the mesh are a few (1-10)microns. However, ultrasonic energy is quickly dissipated in the screen,making it difficult to excite a large screen area ultrasonically.Extended resonators to increase the distribution of ultrasonic energyover the screen are disclosed in EP 0652810. However, for large sieveareas, multiple transducers are still normally required.

It is also known to use guide members located above the screen toimprove the flow of material to be sieved over the surface of thescreen. For example, scroll-shaped guide members are used with circularsieves to ensure material to be sieved moves progressively from thecentre of the screen, where it is first delivered, outwards in agenerally spiral path, covering nearly all regions of the screen surfacebefore reaching the outlet for oversize particles at or near the screenperiphery. This increases the residence time over the screen, tomaximise the opportunity for fines to pass through the screen. Otherguide member shapes and arrangements are used for different sievedesigns, in each case to improve material flow over the screen toincrease the time for undersize to separate from oversize.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided asieve comprising: a base, a sieve screen frame mounted on the base, asieve screen mounted in the frame, a vibrator arranged to vibrate theframe relative to the base, a guide member above the sieve screen forcontrolling flow of material to be sieved over the sieve screen; and anexcitation source arranged to vibrate the guide member so as to induce adeblinding excitation of the sieve screen.

The guide member typically comprises a bar-like member secured to thetop surface of the sieve screen, and shaped to control the flow ofmaterial over the sieve screen as desired. Such an arrangement providesthe advantage that the sieve screen can be excited over an extended areato provide a deblinding effect, whilst at the same time controlling theflow of material over the screen surface. Generally, the level ofdeblinding excitation of the sieve screen decreases with increasingdistance from the source of the excitation. In the above describedaspect of the invention, the highest level of excitation is near theguide member, which is also where the majority of the material to besieved tends to flow. As a result the effectiveness of the sieve can beincreased.

The material to be sieved may be a dry particulate solid or a liquidcontaining solid (or at least non-flowable) parts. In the case of aliquid the guide member can allow an increased head of the liquid to beretained over the sieve screen, which improves throughput rate.

Instead of being secured to the sieve screen, the guide member may beonly in contact with the screen, e.g. pressing against the screen withsufficient pressure to enable vibrations in the guide member to betransmitted to the screen to provide the deblinding excitation.

According to a second aspect of the present invention there is provideda sieve comprising:

-   -   a base, a circular sieve screen frame mounted on the base, a        circular sieve screen mounted in the frame and having a centre,        a vibrator arranged to vibrate the frame relative to the base, a        resonator secured to or contacting the sieve screen, wherein the        resonator takes the form of a spiral-like curve starting near        the centre of the sieve screen, the curve having a progressively        increasing radius of curvature and extending through at least        270° about said centre; and an excitation source arranged to        excite the resonator, to induce a deblinding excitation of the        sieve screen.

The progressive increase in curvature may be continuous or in one ormore steps. This provides the advantage that the excitation is spreadmore effectively over the surface of the screen than with prior artsieves, especially for large diameter sieves.

Although ultrasonic excitation of the sieve screen has been discussedpreviously, the invention is not so limited. For example, the sievescreen may be excited at lower frequencies, or even by hitting ortapping the guide member.

According to a third aspect of the invention, there is provided a sievecomprising: a base, a sieve screen frame mounted on the base, aseparator screen mounted in the frame, a vibrator arranged to vibratethe frame relative to the base, a resonator secured to or contacting theseparator screen, wherein the resonator comprises a rod extendingbetween spaced ends, an ultrasonic transducer at one of said spaced endsto excite the resonator rod at a resonant frequency having apredetermined wavelength along the length of the resonator rod, saidresonator rod having at least a portion of its length which bendssmoothly in a single direction of curvature through at least 90°, andthe rod having a minimum radius of curvature at any point between saidspaced ends which is greater than said predetermined wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a sieve embodying the presentinvention;

FIG. 2 is a plan view of the embodiment of FIG. 1;

FIG. 3 is a plan view of a further embodiment, showing the flow patternof material over the sieve screen surface;

FIG. 4 is a plan view of a still further embodiment, showing the flowpattern of material over the sieve screen surface;

FIG. 5 is a plan view of a still further embodiment;

FIG. 6 is a plan view of a still further embodiment, showing partialflow patterns over the surface of the sieve screen;

FIG. 7 is an enlarged view of a portion of FIG. 6;

FIG. 8 is a plan view of a still further embodiment;

FIG. 9 is a plan view of a still further embodiment, which also showsthe flow pattern of material over the screen surface;

FIG. 10 is a plan view of a still further embodiment;

FIGS. 11 a-d are detailed views of the guide member arrangement of FIG.10;

FIG. 12 is a plan view of a still further embodiment;

FIG. 13 is a scrap cross-sectional view through FIG. 12 showing anenlarged view of the nodal decoupler;

FIG. 14 is a plan view of a still further embodiment;

FIG. 15 shows a cross-sectional view along line A-A of FIG. 14;

FIG. 16 is a cross-sectional view taken along line B-B in FIG. 14.

FIG. 17 is an underneath plan view of a further embodiment of theinvention incorporated in a sieve with a rectangular frame;

FIG. 18 is an underneath plan view of a variation of the embodiment ofFIG. 17; and

FIGS. 19 a to 19 f are schematic illustrations of additionalembodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, this shows a sieve 2 embodying the presentinvention. The sieve 2 comprises a sieve screen 10 in the form of amesh, which is held in a sieve screen frame 6, for example by clamping.The frame 6 and sieve screen 10 may be rectangular but a popularcircular shape is shown in this example.

The sieve screen frame 6 includes an inner support frame 8, which maytake the form of an ‘X’ frame, although it may take other forms. Thesieve screen frame 6 is attached to a lower cylindrical container 7, forexample by clamping. An upper cylindrical container 9 is secured, e.g.also by clamping, on top of the screen frame 6 to act as a containmentwall for the product to be sieved when it is on the sieve screen surface10.

The lower container has a domed floor 22. The lower container is securedon a skirt-shaped annular casting 18, e.g. by clamping.

The sieve also has a fixed base 4 which is attached to the floor 36, inthis embodiment by using sieve stands 38. However, in alternativeembodiments the base may simply stand on a suitable surface, may befixed to a suitable surface or may be arranged on wheeled or othermounts.

The skirt is supported on the fixed base using a suspension support 20.In this particular embodiment the suspension support 20 comprises a rod19 attached to the casting 18 and base 4 using elastomeric bushings 21.This arrangement permits both horizontal and vertical movement of thecasting 18 and therefore of the sieve frame 6 and sieve screen 10. Othermethods may be used for supporting the sieve screen frame on the fixedbase, for example spring mounts.

A motor 23 is mounted on the fixed base 4 and flexibly attached, forexample using a rubber coupling 25, to a vibrator 12. The vibrator 12comprises a bearing housing 29 secured in the centre of the casting 18,a motor shaft 24 which when the motor is at rest is generally vertical,and upper and lower eccentric weights 26, 28. The upper eccentric weight26 is attached to the upper end of the motor shaft 24. The lowereccentric weights 28 are attached to the lower end of the motor shaft24. In this example the mass of the lower weight is greater than that ofthe upper weight. However, the effective eccentricity of the mass of oneor both of the upper and lower weights may be adjustable and therelative angular positions of the two weights on the motor shaft 24 canalso be altered. By altering the effective eccentricity and thepositions of the masses the vibration transmitted using vibrator 12 maybe varied to give optimum sieve performance for particular applications.

In use, vibrator 12 in combination with the suspension mounting of theskirt 18 will result in vibratory motion being imparted to the sievescreen frame 6 and thereby the sieve screen 10, such a motion havingboth horizontal and vertical components.

A guide member 14 is located on the sieve screen surface 10 and theguide member is used to control the flow of the material to be sievedover the sieve screen surface. An excitation source 16 is attached tothe guide member 14 and excites the guide member, preferably so that itmoves in a vertical direction. The guide member 14 thereby preferablydrives a vertical vibration of the sieve screen 10. The excitationsource 16 of this particular embodiment is additionally attached to theX-frame 8 for support. The various methods of excitation and fixationwill be described in more detail subsequently.

For simplicity, how the material to be sieved is supplied to the sieve12 is not shown. However, this may be at any point on the sieve screensurface, but is typically at or near the centre of a circular sieve orat one end of a rectangular sieve.

An outlet 32 for removal of oversized particles is shown and this willremove particles which remain on the sieve screen surface. Onceparticles with a size smaller than the apertures in the sieve screenframe have fallen through these apertures they are directed by the dome22 towards an outlet 30 for fines. The dome 22 serves an additionalpurpose of preventing material which has fallen through the sieve screenfrom fouling the vibrator 12, and in particular the upper eccentricweight 26. Although a dome is depicted in this particular embodiment,this feature may take other forms, for example a cone or a continuousslope across the width of the sieve.

Also shown in this embodiment is a support device 34 which is attachedto the guide member and is supported on the X-frame 8. The forms whichmay be taken by the support device 34 will be discussed subsequently.

FIG. 2 shows a plan view of the sieving apparatus 2 of FIG. 1. The sieve2 has a circular sieve screen frame 6 in which is secured a circularsieve screen 10 and in addition an X-frame 8. On the surface of sievescreen 10 is located the guide member 14. The guide member 14 is securedto the sieve screen, for example using an adhesive. The guide member 14in this embodiment takes the form of a spiral-like shape having an innerend approximately at the centre of the sieve screen 10 and extendingoutwards with a steadily increasing radius of curvature throughapproximately 540°. The guide member 14 is secured to an excitationsource 16 which is located substantially at the centre of the sievescreen 10 and is supported on the X-frame 8. A support device 34 islocated at the opposing end of the guide member 14 to support the guidemember on the sieve screen 10. There may also be other supports of thesame or different type.

In use the vibrator 12 produces a substantially gyratory motion of thesieve screen 10. This movement encourages the flow of the material to besieved outwards from the centre over the sieve screen surface. However,the material may be moved too quickly over the sieve screen surface tothe outside of the screen so that fines can be carried with theoversized particles to the outlet 32, reducing efficiency. The guidemember 14 controls the flow of material over the sieve screen surfaceand thereby increases the residence time of material on the sieve screensurface. This increases the efficiency of the sieve, since there is agreater opportunity for fines to fall through the sieve screenapertures. Although it is known to optimise performance for differentmaterials by adjusting the out-of-balance weights 26 and 28 as mentionedabove, this is a time consuming adjustment. The guide member 14 canensure good sieving performance over a wide range of materials. Theguide member 14 is a bar-like member, typically having an L-shaped orrectangular section presenting sufficient height above the screensurface to restrict or substantially prevent material from crossing overthe guide member during sieving.

As mentioned above, the guide member 14 is excited by excitation source16 to impart deblinding excitation to the sieve screen 10. In apreferred example, which will be described in more detail later, theexcitation source 16 is a source of ultrasonic vibration, and is adaptedto excite the guide member 14 resonantly. In order to be a goodtransmitter of ultrasonic energy, the guide member should be preferablyof metal, such as aluminium or stainless steel. The guide member 14ensures the excitation energy from source 16 is distributed over thescreen 10, to increase the area of the screen 10 which is sufficientlyexcited to provide effective deblinding.

FIGS. 3 and 4 show alternative configurations of the guide member 14. Inthese embodiments the excitation source 16 is located towards theoversize outlet 32. In FIG. 3, the guide member 14 has a circular partextending over about 300° of arc, which is secured to be generallyconcentric in the sieve frame 6. One end of the arc bends outwardstowards the frame 6 to the excitation source 16. In FIG. 4, the circularpart extends over only about 150° of arc. The flow patterns 39 for thematerial being sieved are also shown, from which it can be seen that thematerial enters substantially at the centre of the sieve 2 and movesradially outwards from the point of entry in all directions. The guidemember 14 alters the flow of the material so that it is directed in aspiral-like manner over the sieve screen surface 10. This increasesexposure of the material to the sieve screen and also the time thematerial is resident on the sieve screen surface. Although no supportdevice 34 is shown in either FIG. 3 or 4, if required this may beattached similarly to the guide member 14 as shown in FIG. 2.

FIG. 5 shows a further alternative for the shape of the guide member 14.Again, the guide member 14 has one end substantially at the centre ofthe sieve screen 10. However, in this embodiment the guide member ismade up of inter-connected sections, each section having a constantradius of curvature. The points of interconnection of the sectionsprovide cusp-like formations, which tend to deflect material inwards onthe sieve screen as the material flows around inside the guide member14. This results in the material being exposed to more of the sievescreen surface and gives a greater opportunity for the fines to passthrough the sieve screen aperture.

FIG. 6 shows a further embodiment of the present invention. To assist inmovement of the material across the sieve screen surface cusps 40 areattached to a spiral shaped guide member 14 and also the inner edge ofthe sieve screen frame 6. As shown in more detail in FIG. 7 the cusps 40act in a manner similar to that described in respect of FIG. 5 bydeflecting material inwards as it flows past against the guide member 14or the frame edge. The cusps 40 may be incorporated into the guidemember 14 and sieve screen frame 6 during manufacture, or by theaddition of separate pieces attached by welding, or any other form ofmechanical attachment subsequent to manufacture.

FIG. 8 shows a plan view of an embodiment of the present invention whichis particularly suited for use in wet applications as well as dryapplications. A plurality of separate guide members 14 each have arespective excitation source 16. The guide members have minimal gapsbetween the end of one and the start of the next and together form aspiral shape so that the flow of material is directed over the sievescreen surface as for the single spiral shaped guide member. Themultiple guide members may take other forms as required to controlmaterial flows over the sieve screen. For example, the sections could bestraight sections, particularly for a rectangular sieve. The use ofadditional excitation sources is advantageous, particularly though notexclusively for wet applications, whenever a greater amount of energy isrequired for deblinding excitation of the sieve screen 10, e.g. tocounter increased damping.

The previously described embodiments of the present invention have beencircular sieves. However, the present invention is also applicable torectangular sieves, and examples are shown in FIGS. 9 and 10. In theseexamples, the action of the sieve tends to transport material over thesieve screen from one end to the other, e.g. top to bottom in thedrawings. Guide members 14 directs the flow in a path traversing thesieve from one side to the other, maximising the sieve screen surfacecovered and residence time on the sieve screen. As before, the guidemember 14 may be multiple with respective excitation sources 16, asshown in FIG. 9, or there may be a single zig-zag guide member with asingle excitation source 16 as shown in FIG. 10. In the latter case theflow path must pass through the guide member and FIGS. 11 a to dillustrate two methods in which this may be achieved. FIG. 11 a shows abridge 41 formed in the guide member where a portion of the guide member14 is raised to form an opening for the material to flow through. FIGS.11 b and c show a guide member 14 which has a T-cross section, and fromwhich a portion 43 is removed to provide an opening for the product toflow through. In another embodiment, shown in FIG. 11 d, the guidemember 14 has multiple openings 45 along its length.

FIG. 12 shows an embodiment similar to that of FIGS. 1 and 2 in whichthe spiral shaped guide member 14 is driven ultrasonically by acentrally mounted excitation source 16. The guide member 14 is supportedpart way along its length and at its outer end by respective supportingdevices 34 a and 34 b. The device 34 a is further illustrated in scrapsection in FIG. 13 and will be described in detail below with referenceto FIG. 16.

As has been previously mentioned, the guide member may be ultrasonicallyexcited, commonly at frequencies above 20 KHz. FIG. 16 provides adetailed illustration of an excitation source 16 configured to provideultrasonic excitation and a support device 34 which is suitable for usewith ultrasonic frequencies.

The excitation source comprises a transducer 42 for convertingelectrical energy to ultrasonic wave energy, for example by using thepiezoelectric effect. The transducer may be a half wave stack-typetransducer of a kind which will be familiar to those experienced inultrasonics. A circular resonator boss 44 is attached to the active endof the transducer 42. The resonator 44 converts the longitudinalvibration of the transducer to a transverse diaphragm mode. Theexcitation source 16 is supported on the X-frame 8 by the use of acentral support 48. The dimensions of the central support 48 are chosensuch that it is one half wavelength in length so that a node is formedat a point about half way along the length of the central support 48. Acylindrical sleeve 50 is attached to the support 48 at the node point,and the sleeve 50 is secured to the X-frame 8, for example by welding.Because the connection to the central support is at a node, the mountingarrangement decouples the transducer 42 from the X-frame 8, minimisingloss of ultrasonic energy to the frame.

The resonator 44 is attached at its outer periphery to the guide member14 to transmit ultrasonic energy to the guide member. The dimensions ofthe guide member 14 are preferably chosen so that the length isapproximately a whole number of half wavelengths, so that the guidemember 14 can be driven in resonance to maximise the transfer ofultrasonic energy from the transducer 42 into the guide member 14.However, the guide member 14 would normally be a substantial number ofhalf wavelengths long. Therefore, it is not necessary to make the guidemember to have a length precisely equal to a whole number of halfwavelengths, as it can readily be brought into resonance by a smallchange in the drive frequency of the transducer 42, without great lossof efficiency. Also, in some applications, vibration of the guide member14 may be damped, e.g. by the loading of the sieve screen and materialto be sieved, to such a degree that little vibration energy is reflectedat the far end of the member. Then, the guide member functions as anon-resonant transmission member rather than as a resonator.

Although resonator boss 44 is illustrated interconnecting the transducer42 and the guide member 14, in some applications it may be satisfactoryto connect the transducer 42 directly to the guide member 14 or througha different coupling system.

Also shown in FIG. 16 is a support device 34 (corresponding to device 34b in FIG. 12) designed to support the guide member 14 on the sievescreen 10. At ultrasonic frequencies it is preferable to provide asupport device 34 which ultrasonically decouples the guide member 14from the support frame, to which it is attached.

Accordingly, the support device 34 comprises a cylindrical resonatorboss 52, that may be similar to boss 44, which is attached to the guidemember 14, so that a diaphragm mode of vibration is excited in boss 52.At least one diaphragm mode node is therefore formed at a predictableposition on the resonator boss 52. Decoupling washers 54 a, 54 b haveskirts which are located against the upper and lower surfaces 52 a, 52b, of the resonator boss 52, at the diaphragm mode node. Thesedecoupling washers 54 a and 54 b therefore experience minimalexcitation. A support bracket 58 welded to the X-frame 8 engages thelower decoupling washer 54 b. A bolt 60 is used to clamp the resonatorboss 52 between the washers 54 a and 54 b and the support flange 58 tosecure the boss to the X-frame 8. The bolt 60 extends through anoversize hole in the resonator boss 52, so as not to contact the body ofthe resonator boss 52. This configuration effectively decouples theguide member 14 from the X-frame 8, since the only point of contact withthe resonator boss 52 is at the diaphragm mode node, i.e. a point ofminimum vibration. This nodal decoupling boss is also described inGB-A-2343392. A similar construction is used for the support device 34 aof FIGS. 12 and 13.

The boss 52 may be excited to resonate in other modes, provided thepoint or points of contact with the boss are made at appropriate nodalpoints of the resonant mode to ensure decoupling.

FIGS. 14 and 15 show an alternative supporting arrangement for the guidemember 14. FIG. 15 shows flange 62 in the form of an inverted J, whichis attached to the X-frame 8 and to the guide member 14. Although thisconstruction of support provides less effective ultrasonic decoupling ofthe guide member 14 from the X-frame 8, this may be sufficient for manypurposes, provided the area of contact with the guide member 14 is smallcompared to a quarter wavelength of the resonant vibration of the member14.

As has been previously mentioned, the excitation may be at variousdifferent frequencies and the excitation source may comprise a number ofalternatives. For example, instead of using an ultrasonic transducer theexcitation source 16 may comprise a pneumatic actuator vibrating theguide member 14 at lower frequencies, e.g. several tens or hundreds ofhertz. This is particularly advantageous in applications where the useof an electrically powered actuator may pose a fire or explosion risk. Asuitable pneumatic actuator is described in International PatentApplication WO 03/024626. The pneumatic vibrator may provide animpulse-type excitation of the guide member, e.g. by means of areciprocating mass in the pneumatic actuator, to cause high frequencyresonant vibrations (or ringing) of the guide member.

Alternatively, electrically powered actuators may be used to providelower frequency excitation.

Therefore in summary, mechanical, electro-mechanical, pneumatic andother forms of actuators may be used in the excitation source ofembodiments of the present invention. Particularly at lower excitationfrequencies it may not be necessary to excite the guide member atresonance, and the above described arrangements for decouplingexcitation energy at the supports for the guide member and/or thetransducer may also not be required.

Although in the previously described embodiments the excitation sourceis directly coupled to the guide member, in other embodiments theexcitation source may not be permanently connected to the guide member,but may instead have a striking surface arranged to strike the guidemember when the excitation source is energised. Also, the excitationsource may be parasitic, that is dependent on the primary sieving actionof the sieve frame. For example, the excitation source may comprise oneor more free or resiliently mounted parasitic bodies which are caused tomove by the primary sieving action and to strike the guide member toproduce the required deblinding excitation. Striking of the guidemember, either by a separately energised actuator or by a parasiticbody, may induce resonant high frequency ringing of the guide member.

Although the excitation source or transducer is shown in the previouslydiscussed embodiments as being supported on an ‘X’ frame, the excitationsource may in fact be wholly supported by the screen, or may besupported at least partially by a flexible or rigid coupling to theframe or the fixed base.

The “sieve screen” may comprise a number of layers, for example it maycomprise a first screen and second screen arranged above and supportedby the first. In such multi-screen sieves, one or more of the guidemembers arranged on the screen may be directly excited by the excitationsource.

In all the embodiments described above, a guide member is fastened tothe top of the sieve screen in order to control the flow of material tobe sieved over the screen surface, as well as to provide for aneffective deblinding excitation of the screen itself. In a furtherembodiment, a spiral shaped resonator is fastened beneath the screen.FIG. 2 of the drawings is also a schematic representation of thisembodiment, except that the spiral resonator 14 illustrated in thedrawing is secured beneath the sieve screen rather than on top. Thespiral shape may have a continuously increasing radius of curvature (asin FIG. 2) or the radius may increase in one or more steps. Further theresonator 14 need not have a profile designed to provide a gooddeflecting action as is necessary when acting as a guide member on topof the screen. Instead, the resonator 14 may be a simple rectangularsection tube or solid bar, or else may have a strap shape having alarger dimension secured to the screen. In each case, the resonator 14should preferably be made of metal or of another material which is anexcellent propagator of acoustic energy.

The resonator 14 is excited by an ultrasonic transducer connected to theresonator 14 at the centre of the spiral as shown as 16 in FIG. 2. Againthe transducer and the spiral may be supported on an X-frame 8 beneaththe sieve screen by decoupling arrangements as illustrated in FIG. 16,except that the resonant bosses 44 and 52 shown in FIG. 16 would be alsolocated beneath the sieve screen.

The spiral resonator 14 is driven to resonance so that deblindingexcitation is distributed over the sieve screen to increase the area ofthe sieve screen which is effectively excited so that blinding can beminimised. In order to provide effective distribution of the ultrasonicenergy over the sieve screen area, the spiral should extend through atleast 270° of arc, and preferably more than 360° of arc, as illustratedin FIG. 2.

Importantly, the spiral design can allow deblinding excitation to bedistributed to a screen of larger sizes by increasing the number ofturns of the spiral. In this way almost any practical screen size can beexcited using a single length of resonator driven by a singletransducer. This avoids the problems of tuning the different lengths ofa multiple rod resonator to the same driving frequency, and theadditional complication of using multiple single rod resonators withrespective separate transducers.

Although the spiral resonator designs of FIGS. 2, 12 and 14 have thespiral starting at the centre of a circular sieve, it may be preferredto locate the inner end of the spiral away from the centre. It is commonfor material to be screened to be delivered to the centre of the screen,so that keeping this region clear can be beneficial.

The above advantages may also be obtained with other curved rodresonator designs. By using a gently curved rod resonator secured to thescreen, ultrasonic energy can be distributed over the area of a screen,thereby reducing or eliminating the regions of the screen which receiveinsufficient ultrasonic energy to ensure deblinding during sievingoperations. Using a rod resonator extending between spaced ends, andexcited by an ultrasonic transducer at one of the spaced ends, resonanceof the rod over its entire length can usually be ensured. By providingthe rod with a gently curved shape, the ultrasonic energy can bedelivered efficiently to all parts of a sieve screen. In order toachieve the appropriate coverage of a sieve screen, the rod should haveat least one portion of its length which bends smoothly and in a singledirection of curvature through at least 90°. Furthermore, the entirelength of the rod should comprise smoothly blended curved or straightportions so that the minimum radius of curvature at any point betweenthe ends of the rod is greater than the wavelength of ultrasonic energyin the rod at the resonant frequency at which the rod is excited.Sharper bends tend to reduce the efficiency with which ultrasonic energycan travel along the rod around the bend and can give rise toreflections of ultrasonic energy at the bend, so that different parts ofthe length of the rod may prefer to resonate at different frequencies.By forming the rod with smoothly blended components and gentle curves,the whole length of the rod normally acts as a single resonator withultrasonic energy distributed along the entire length.

Better coverage of the area of screen can be obtained if the curvatureof the rod varies over the length of the rod to form a more complexcurved shape each as a spiral, a serpentine or S-shape, or a blendedcombination of straight lines and curves.

In practice, the ultrasonic transducer may be operated to excite the rodresonator at a resonant frequency between 18 kHz and 40 kHz. A preferredoperating frequency is about 35 kHz. The corresponding wavelength ofultrasonic energy along the length of the resonator rod is between 25 mmand 35 mm and typically about 30 mm. In most applications, the minimumradius of curvature of a resonator rod should be greater than 50 mm, andpreferably greater than 100 mm.

Although the rod should have at least one portion bending smoothly witha single direction of curvature by at least 90°, effective coverage of ascreen surface can often more easily be achieved with a rod which bendswith a single direction of curvature by at least 180°. It should beunderstood that a portion of a rod that bends with a single direction ofcurvature may include a straight line portion separating two curvedportions bending in the same direction. The rod portion bending in asingle direction of curvature can also be described as having amonotonically changing angle with distance along the portion. This isreferred to herein as a monotonically bending or curving portion.

An example of a smoothly curved rod resonator (other than a spiral) isillustrated in FIG. 17. A rectangular sieve frame 70 is illustratedviewed from beneath. The sieve frame 70 carries a rectangular sieve meshwhich is omitted in this drawing for clarity. The rectangular sieveframe 70 is braced by struts 71 and 72 extending between the long sidesof the rectangular frame so as to be spaced beneath the mesh supportedby the frame. An S-shaped resonator rod 73 is bonded to the underside ofthe screen mesh and is supported at each end by de-coupling mounts 74and 75. The de-coupling mounts 74 and 75 may comprise a circularresonator boss bonded to each end of the resonator rod 73 and sized toresonate in diaphragm mode at a preferred resonant frequency of the rod73. Annular de-coupling extensions are bonded to the bosses at diaphragmmode antinodes, to provide mounting points for attachment to brackets 76and 77 secured to the frame 70. Accordingly, the de-coupling mounts atthe ends of the rod 73 may correspond to mounts 34 illustrated in FIG.16, and also to the de-couplers described in GB-A-2343392.

An ultrasonic transducer is connected to the de-coupling boss 74 toexcite the rod 73 along its entire length at a resonant frequency. Thewavelength of ultrasonic energy along the rod at this resonant frequencyis typically about 30 mm.

As illustrated in FIG. 17, the rod 73 comprises a first monotonicallycurving portion 78 which bends through about 210°, smoothly connected toa straight portion 79, which is in turn smoothly connected to a furthercurved portion 80 which also bends monotonically (with curvature ofopposite sign to the first curved portion 78) back through about 210° tothe termination boss at mounting 75. The radius of curvature of each ofthe curved portions 78 and 80 is about 300 mm.

As can be seen from the Figure, the illustrated design providesexcellent coverage of the rectangular screen 70, so that no part of thescreen surface is more than about 400 mm from a source of ultrasonicenergy, even though the sieve mesh itself is about 1 metre wide andabout 2 metres long.

FIG. 18 illustrates a further example applied to a shorter rectangularsieve frame of about 1 metre by 1.4 metres. In this illustration, thesame reference numerals have been used to indicate corresponding partsas for the embodiment of FIG. 17. However, the rod resonator 73essentially comprises only the first curved portion 78 directly blendedinto the last curved portion 80, with the intermediate straight portion79 of the FIG. 17 embodiment removed. Each of the curved portions 78 and80 in FIG. 18 have a radius of curvature of about 250 mm.

FIGS. 19 a to 19 f illustrate further curved rod resonator designsfalling within the scope of the invention. In FIG. 19 a, the rod bendsmonotonically between the two ends by a total of about 360°. In FIG. 19b, the rod bends by a total of about 310° monotonically from one end tothe opposite end. In FIG. 19 c, there is a first portion which bendsmonotonically through about 270°, smoothly blended with a secondportions which bends monotonically in the opposite direction also byabout 270°.

FIG. 19 d illustrates a generally rectangular form with radiusedcorners, so that the rod bends monotonically from one end to theopposite end through a total of 360°. The radius of curvature of thecorners should typically be about 100 mm.

FIG. 19 e illustrates an S-shaped rod comprising a first portion whichbends through a 270° monotonically in three 90° curves interconnected bystraight portions. The rod then bends 270° monotonically in the oppositedirection again by three bends interconnected by straight portions. Thebend radius of curvature is again about 100 mm. In FIG. 19 f an S-shapeis illustrated having a first portion bending monotonically throughabout 120° smoothly connected to a straight diagonal portion, and inturn smoothly connected to a further curved portion bendingmonotonically in the opposite direction again by about 210°.

For all the above described resonators, both spiral and other curvedshapes, the transducer may be located at either end of the resonator.

Other possible excitation sources for the resonator 14 include strikingsources, both active (driven) and passive (parasitic), which applyimpulses producing resonant ringing vibration of the resonator 14.

Although the term resonator is used for the resonator 14 in thisembodiment, the member may function more as a transmission member forthe vibration energy transmitted to the member from a driven excitationsource.

Embodiments of the invention may be applied also to sieves with multiplescreens, for example multi layer screens with lower screens ofincreasing fineness for classifying materials into more than twoparticle sizes. Then one or more of the screens of the sieve may befitted with the excited guide member, or the spiral or smoothly curvedresonator, as described above.

It should also be understood that the generally spiral-shaped guidemembers or resonators in various of the examples described above neednot have an inner end at the centre of a circular sieve screen.

In a further example, a so-called cascade sieve has upper and lowerscreens of the same mesh, with oversize from the upper screen being fedon to the lower screen to retrieve remaining fines which may not havehad an opportunity to pass through the upper screen. Fines which do passthrough the upper screen are collected and tunnelled through an aperturein the centre of the lower screen. In such a cascade sieve design, thelower screen can be fitted with an excited guide member or a spiralresonator having an inner end terminating outside the central apertureof the lower screen.

The excitation induced in the guide member in the embodiments of theinvention described above has been referred to as one which produces adeblinding excitation in the sieve screen. Generally, secondaryexcitation of the sieve screen, e.g. at ultrasonic frequencies, is knownto speed up the flow of fines through the screen during sieving so thatthe productivity of the sieve is improved. This enhanced flow throughthe screen may be the result of other processes than the removal ofblind areas on the screen, such as the fluidisation of the material atthe screen interface. It should be understood that the term deblindingused herein to describe the excitation applied to the screen is intendedto encompass other processes by which the excitation enhances productflow rate through the screen compared to the rate achieved with only thebasic vibratory sieve action.

In the above described examples of the invention, the guide member orthe resonator is described as being secured to the sieve screen. Inother embodiments, the guide member may be only in contact with thescreen, e.g. pressing against the screen with sufficient pressure toenable vibrations in the guide member to be transmitted to the screen toprovide the deblinding excitation. Where the embodiment provides only aresonator which does not necessarily act as a guide member, i.e. onewhich may be located beneath the sieve screen, the resonator again maybe only in contact with the screen and not specifically secured to it.

Further, in examples of the invention which are intended primarily forsieving (or straining) liquid materials, the guide member may be spacedabove the sieve screen so as to make no direct contact with the screenover at least a part of the length of the guide member. Then, providedthere is sufficient depth above the sieve screen of liquid to be sievedso as to fill the gap between the guide member and the sieve screenitself, vibrations in the guide member are transmitted to the screen toprovide the deblinding excitation through the liquid material. Similartransmissions may also be possible through some dry materials. Inpractical arrangements, the spacing between the guide member and theliquid material should not be so great or so extensive as to provide noeffective control over the flow of the liquid material over the sievescreen. When the space between the guide member and the sieve screen isonly a fraction of the head of liquid to be sieved which may be retainedover the sieve screen, the guide member still provides effective controlof the flow of the material to be sieved over the top of the sievescreen and simultaneously enables deblinding excitation (as definedabove) to be transmitted to the sieve screen. When the guide member isnot in contact with or secured to the sieve screen over its entirelength, the guide member may be mounted directly to the sieve screenframe or sieve deck, preferably by appropriate acoustic decouplingmounts, to minimise loss to the sieve screen frame of vibration energysupplied to the guide member for use in inducing deblinding excitationof the sieve screen.

In a yet further example of the invention using a guide member on top ofthe sieve screen, the guide member may form a substantially closed loop,for example a circle, which may be located concentrically in a circularsieve frame for example. Then, the guide member may have multipleapertures through the guide member, for example in the mannerillustrated in FIG. 11 d, to permit material being sieved to flowoutwards from within the closed loop guide member. The presence of theguide member, together with the apertures through it, have the effect ofcontrolling the outwards flow of material to be sieved under the mainvibratory action of the sieve frame and screen. In this way, theresidence time of material to be sieved over the sieve screen can beincreased, to improve the chances of fines reaching the sieve screen andfalling through. In this way, sieving efficiency can be increased whilstat the same time ensuring good deblinding excitation of the screen.

1. A sieve comprising: a base; a sieve screen frame mounted on the base;a sieve screen mounted in the frame; a vibrator arranged to vibrate theframe relative to the base; a guide member above the sieve screen forcontrolling flow of material to be sieved over the sieve screen; and anexcitation source arranged to vibrate the guide member so as to induce adeblinding excitation of the sieve screen.
 2. A sieve in accordance withclaim 1, wherein the excitation source is attached to the guide member.3. A sieve in accordance with claim 1, wherein the sieve screen frameand sieve screen are circular.
 4. A sieve in accordance with claim 2,wherein the guide member takes the form of a spiral-like curve having aprogressively increasing radius of curvature and extending through atleast 270°.
 5. A sieve in accordance with claim 1, wherein the sievescreen frame and sieve screen are rectangular.
 6. A sieve in accordancewith claim 5, wherein the guide member is a single zig-zag-shaped rodhaving at least one aperture above the sieve screen through whichmaterial to be sieved can flow.
 7. A sieve in accordance with claim 1,having a plurality of said guide members, each having a respective saidexcitation source.
 8. A sieve in accordance with claim 1, wherein theguide member is secured to the top surface of the sieve screen.
 9. Asieve in accordance with claim 1, wherein the guide member is in contactwith the top surface of the sieve screen.
 10. A sieve in accordance withclaim 1 particularly for sieving a liquid material, wherein the guidemember is spaced from the top surface of the sieve screen and thedeblinding excitation is transmitted to the sieve screen through saidliquid material.
 11. A sieve comprising: a base; a circular sieve screenframe mounted on the base; a circular sieve screen mounted in the frameand having a centre; a vibrator arranged to vibrate the frame relativeto the base; a resonator secured to or contacting the sieve screen,wherein the resonator takes the form of a spiral-like curve starting ator near the centre of the sieve screen, the curve having a progressivelyincreasing radius of curvature and extending through at least 270° aboutsaid centre; and an excitation source arranged to excite the resonator,to induce a deblinding excitation of the sieve screen.
 12. A sieve inaccordance with claim 1, wherein the excitation source comprises apneumatic actuator.
 13. A sieve in accordance with claim 1, wherein theexcitation source comprises an electrically powered actuator.
 14. Asieve in accordance with claim 1, wherein the excitation source providesultrasonic excitation.
 15. A sieve comprising: a base; a sieve screenframe mounted on the base; a separator screen mounted in the frame; avibrator arranged to vibrate the frame relative to the base; a resonatorsecured to or contacting the separator screen, wherein the resonatorcomprises a rod extending between spaced ends; an ultrasonic transducerat one of said spaced ends to excite the resonator rod at a resonantfrequency having a predetermined wavelength along the length of theresonator rod; said resonator rod having at least a portion of itslength which bends smoothly in a single direction of curvature throughat least 90°, and the rod having a minimum radius of curvature at anypoint between said spaced ends which is greater than said predeterminedwavelength.
 16. A sieve in accordance with claim 15, wherein saidminimum radius of curvature is greater than 50 mm.
 17. A sieve inaccordance with claim 15, wherein said predetermined wavelength isbetween 25 mm and 35 mm.
 18. A sieve in accordance with claim 15,wherein said rod bends in said single direction of curvature, over atleast a portion thereof, by at least 180°.
 19. A sieve in accordancewith claim 15, wherein the sieve further comprises a support framebeneath the sieve screen.
 20. A sieve in accordance with claim 19,wherein said excitation source comprises a transducer, resonator, and asupport device, which supports the excitation source on the supportframe and also acts to minimise the excitation of said support frame.21. A sieve in accordance with claim 20, wherein an additional supportdevice for the resonator is provided at a node and is attached to theresonator such that excitation of the support frame is minimized.
 22. Asieve in accordance with claim 15 including a plurality of saidresontator rods on a single said screen, each of said plurality ofresontator rods having a respective ultrasonic transducer at one end ofthe rod.
 23. A sieve in accordance with claim 15, wherein the curvatureof the rod varies over the length of the rod between the ends.
 24. Asieve in accordance with claim 1, wherein the excitation source is notattached to the guide member or resonator and has a striking surfacearranged to strike the guide member or resonator when the excitationsource is energized.
 25. A sieve in accordance with claim 1, wherein theexcitation source is not attached to the guide member or resonator andhas a contact surface arranged to apply pressure to the guide member orresonator to communicate vibrations to the guide member or resonatorwhen the excitation source is energized.
 26. A sieve in accordance withclaim 1, wherein the excitation source is parasitic, depending on thevibration of the frame produced by said vibrator.
 27. (canceled)